The present invention relates to a mechanism for correcting light distribution and light amount applied to a projecting mechanism in a copying machine.
In a projecting mechanism in an electro-photographic copying machine a lens is indispensable for forming an image projected on a photo-sensitive member as an image-carrier. However, the lens has the characteristic of a "cos.sup.4 .theta. low" (whose details will be described later) that "a luminous flux density at an image point aside from an optical axis is reduced in proportion to cos.sup.4 .theta. of the luminous flux density at an image point on the optical axis."
Accordingly, an unevenness of exposure amount occurs, i.e., the image-forming surface of the photo-sensitive member shows a bright exposure distribution in the vicinity of the center of the optical axis and a dark exposure distribution in the peripheral area thereof.
According to the present invention, the distribution of luminous flux density on a cross section of the optical path vertical to the optical axis is herein after referred to as light distribution. Exposure amount is expressed by the product of the quantity of luminous flux incident upon an image-forming surface (photo-sensitive surface) from a light source per a unit area and an exposure time, namely, lux.sec (CMS).
In order to solve the above-described problem, electrophotographic copying machine capable of varying projection magnification, as disclosed in Japanese Patent Publication Open to Public Inspection No. 146630/1977 (hereinafter referred to as Japanese Patent O.P.I. Publication), one side of a slit as an optical path regulating means is provided so as to be moved forward and backward relative to an optical path so that light distribution is controlled. Japanese Patent O.P.I. Publication No. 73767/1982 disclosed an optical cut off means which provided in addition to a slit so as to be moved forward and backward perpendicular to an optical axis of a lens so that light distribution is controlled.
However, since the optical cut off means capable of going forward and backward relative to the optical path has a fixed shape, these mechanisms are incapable of achieving a superior light distribution correction for all of a large number of projection magnifications. In particular, if there is a region where an optical cut off plate does not exist in the longitudinal direction of the cross section of the optical path formed by the slit, i.e., the optical cut off amount is discontinuous in the longitudinal direction, portions having much light amount are locally formed.
In addition, if an optical path 1 is shifted up and down (in a direction shown by an arrow in FIG. 27) relative to an optical cut off means 99, as shown in FIG. 27, the out offlight amount is remarkably changed.
Furthermore, since a magnification is minified, the cross sectional shape of an optical path is reduced in both its width and length, the optical cut off means must move by the minified amount and further by the quantity required for light distribution correction, which causes the mechanism to be complicated.
Besides, when a reduction magnification is relatively small, the cross sectional shape of the optical path is reduced in both its width and length. In order to accomplish a reasonable light distribution correction, the movement quantity of the optical cut off means must be more accurate than that in an equal size copying and a magnified copying.
In addition, in a mechanism of which an optical cut off means moves linearly, a link mechanism provided with a guide rail and many knots is required, and as such, the construction is complicated.
Furthermore, since conventional light distribution correction mechanisms are always under a partially optical cut off condition, light radiated from a light source is not effectively utilized. Hence uneconomical.
The mechanisms having the above-described problems have been proposed in Japanese Patent O.P.I. Publication No. 136845/1979, Japanese Patent O.P.I. Publication No. 92348/1982, Japanese Patent O.P.I. Publication No. 154265/1982, Japanese Patent O.P.I. Publication No. 134226/1985, Japanese Patent O.P.I. Publication No. 80828/1985 and the like in addition to the above described Japanese Patent O.P.I. Publication Nos. 146630/1977 and 73767/1982.
On the other hand, an optical correction device, in which a rectangular optical cut off plate is provided so as to be at a right angle with an optical axis and the optical cut off plate is adapted to be rotatable in front and rear directions of the optical axis, has been proposed in Japanese Utility Model Open to Public Inspection Publication No. 121953/1982. This art is advantageous in that light amount is hardly lost when light is not cut off, but disadvantageous in that light distribution correction can not be favorably carried out.
In addition to the above-described problem of correcting the unevenness of the light distribution on the same cross section perpendicular to the optical path, there is further a problem that an exposure amount on a photoreceptor fluctuates depending on a variation of a selected magnification as stated in Japan Patent O.P.I. Publication No. 156616/1977.
Namely, due to the reasons why, according to the variation of the magnification, the projected size of the original on the photoreceptor is varied, and the scanning speed for the original is also varied if assuming a photoreceptor speed being constant, the relavant exposure amount on the photoreceptor is varied in accordance with the variation of the magnification as shown in FIG. 27-b.
FIG. 27-b represents the ratio of exposure amount of respective magnification to that of equal size magnification (m=1.0).
It may be well understood from this drawing if comparing with the exposure amount of the equal size magnification, that of the reducing and enlarging magnification have indications respectively the reducing tendency.
For the purposes of maintaining the exposure amount at constant in spite of the variation of the magnification, it may be considered to change the light amount of the light source in accordance with the variation or to change the speed of the photoreceptor.
However, there is a disadvantage that these methods cause the apparatus to be complicated.
As a method of regulating an exposure amount, Japanese Utility Model O.P.I. Publication disclosed to use two pieces of plates which are pivotally connected around a shaft, arranged the one of them being at upper side of the shaft and the other one being at lower side, have symmetrically shaped-opening with each other across the shaft and form a slit by being combined the respective openings.
In the above-mentioned method, both corrections for light distribution and exposure amount may be tried to be carried out simultaneously by inclining the two pieces plates at a same turning angle around the shaft.
However, the magnifications required to be corrected its light distribution may be varied depending on the given light distribution of the light source, while the magnifications required to be corrected its exposure amount may be determined, without the dependence on the light distribution of the light source, in accordance with the characteristics in which the exposure amount may show its peak value at the equal size magnification and decrease its value at both sides of the peak point corresponding to the decreasing of the reducing magnification and the increasing of the enlarging magnification.
Therefore, when varying the magnification, it may be difficult to attain a sufficient correction for both light distribution and exposure amount according to the above-mentioned method, from the view of performing the both correction simultaneously.